In the manufacture of microelectronic devices such as, for example, integrated circuits, it is often necessary to deposit smooth, uniform layers of materials such as metals or semiconductors on a microelectronic substrate. Amorphous, polycrystalline and monocrystalline layers must often be deposited. In addition, such deposition must sometimes be made through patterned openings in a masking layer applied to the substrate, and without depositing any material on the surface of the masking layer. The substrate may be a semiconductor, a metal or any other material. Thus a recurring problem in this field is how to uniformly deposit such materials and how to do so only at target locations on the substrate surface.
Selectivity can be obtained to a degree where substitution, or displacement reactions occur, i.e. reactions where the exposed substrate material is actually chemically displaced by the material being deposited using chemical vapor deposition methods. Due to the nature of such reactions, however, the deposition obtained generally self-terminates once the exposed surfaces of the substrate become overcoated. The result is a deposited layer generally on the order of 100-1000 .ANG. thick. Because of this inherent self-termination, substitution or displacement reactions do not provide a viable process for deposition to arbitrary thicknesses as is often required.
Another known approach to these problems has been the use of chemical vapor deposition (hereinafter "CVD") in which displacement reactions may or may not take place. Selectively is sought via modifying the sticking coefficients and the masking materials and substrates as best as one is able to. In such processes, the material to be deposited is first chemically vaporized, i.e. chemically converted to vapor-phase species, and then transported through vapor into contact with the substrate. Once these vapor-phase species are exposed to the substrate, conditions are manipulated, almost always by a temperature change, such that the vapor-phase species re-react to chemically condense, or deposit, the desired material. The resultant deposited layer generally blankets the substrate surface. Selectivity obtained by such deposition processes is not controllable to the degree required.
In order to counteract these problems and to enhance selectivity, modified two-stage deposition and etching processes have been developed. In such modified processes, the desired material is deposited on the substrate during a first stage of the process and then etched therefrom in a second stage. Sometimes, these first and second stages are simultaneously carried out. In either event, due to a slight preference for unmasked portions of the substrate during the deposition stage and a more enhanced preference for the masked portions of the substrate during the etching stage, the overall process results in net selective area deposition at the unmasked areas of the substrate.
One known example of such a modified process is disclosed for tungsten deposition in U.S. Pat. No. 4,617,087 to Iyer, et al. The Iyer, et al. process begins with the following substitution reaction: EQU 2WF.sub.6 (v)+3Si(s).revreaction.2W(s)+3SiF.sub.4 (v)
This reaction is selective inasmuch as the gaseous tungsten hexafluoride reacts directly with the exposed areas of the silicon substrate to displace the exposed silicon with deposited tungsten. Once the exposed substrate becomes overcoated, however, the reaction self terminates leaving approximately 100-1000 .ANG. of tungsten on the exposed portions of the substrate.
Once this reaction terminates, hydrogen already present in the reaction chamber causes the following hydrogen reduction reaction to become dominant resulting in further tungsten deposition: EQU WF.sub.6 (v)+3H.sub.2 (v).revreaction.W(s)+6HF(v)
Unlike the above described substitution reaction, the hydrogen reduction reaction is generally non-selective, although some selectivity can be obtained depending on the nucleating characteristics of the substrate material vis-a-vis that of the masking layer.
To counteract the unwanted nucleation of tungsten on the masking layer, Iyer, et al. propose that a third, etching reaction be simultaneously initiated within the reaction system. The etching reaction requires the addition of yet another reactant gas, NF.sub.3, into the reaction system. This reaction volatilizes the already deposited tungsten according to the following equations: EQU 2NF.sub.3 (v).revreaction.N.sub.2 (v)+3F.sub.2 (v) (1) EQU 3F.sub.2 (v)+W(s).revreaction.WF.sub.6 (v) (2)
The first of these reactions additionally requires that a plasma source be included within the reactor system.
Because the etching reaction is carried out simultaneously with the deposition reaction, any unwanted tungsten nuclei formed on the surface of the masking layer are immediately volatilized. Nuclei formed on the unmasked surfaces of the substrate are etched to a lesser extent due to their decreased surface areas and the relatively greater sticking coefficient between such nuclei and the tungsten layer already deposited via the substitution reaction. Thus, the overall process results in the desired selectivity. In addition, due to the way in which the deposited layer is formed, uniform layers are obtained.
Several drawbacks or disadvantages, however, are associated with the Iyer, et al. approach. A first is that this method requires the precise control of multiple component sources to feed the involved reactions. In addition, this process involves the expense and burden of maintaining a plasma source within the reactor assembly to fuel the etching phase thereof.
In U.S. Pat. No. 5,037,775 to Reisman, one of the present inventors, and commonly assigned with the present application, an improved selective area deposition process is disclosed for materials such as silicon, germanium and titanium. The Reisman process addresses several of the shortcomings of the Iyer, et al. and other similar selective CVD methods. In the disclosed method, two rather than three reactions are employed to selectively deposit the desired materials. The process involves flowing a gas of a reducible compound of the material to be deposited and a reducing gas into a deposition chamber whereby the gases react to deposit the desired material on the substrate. The reaction is preferential to the exposed areas of the substrate due as indicated, initially in some instances to a displacement reaction, and subsequently the differences in the nucleation characteristics of the masking layer versus the exposed substrate. Some unwanted nucleation of the deposited material on the surface of the masking layer, however, sooner or later obtains. To remove this unwanted nucleation, the flow of the reducing gas is interrupted, causing a second, embedded, etching reaction to become dominant within the reactor. In this reaction, the reducible compound reacts with the deposited material in a disproportionation reaction to convert the deposited material to a volatile species under the experimental conditions employed. The flow of the reducing gas is thereafter restarted and deposition resumes. This two stage cycle is repeated to alternatingly deposit and etch the deposited material on and from the substrate, respectively in a periodic cyclic fashion. Uniquely, the same reducible compound of the deposited material is employed to both deposit and then etch the desired material, thus reducing the number of reactants involved in the overall system.
An example of the Reisman method can be described for silicon deposition wherein silicon tetrachloride is employed in the following hydrogen reduction reaction: EQU SiCl.sub.4 (v)+2H.sub.2 (v).revreaction.Si(s)+4HCl(v)
As long as a sufficient supply of hydrogen is available within the deposition chamber, the reaction will proceed to the right as written, according to the equilibrium constant for the reaction. Coincidentally, because the standard enthalpy of the reaction is positive, the reaction is also driven to the right with increasing temperature. As the reaction proceeds in this way, silicon is deposited on the substrate. However, when the flow of hydrogen is interrupted and the remaining hydrogen becomes mostly depleted, a second, embedded, etching reaction, becomes dominant: EQU SiCl.sub.4 (v)+Si(s).revreaction.2SiCl.sub.2 (v)
Thus, by cyclically modulating the flow of hydrogen into the deposition chamber, the respective dominance of the deposition and etching reactions can be controlled within the reactor. The etch rate of the deposited nuclei (when they are small) from the masked areas is greater than that from the unmasked areas on the substrate because of the higher surface energy and greater surface exposure of the unwanted nuclei. Consequently, the cyclical process results in a net preferential deposition in the unmasked areas of the substrate.
This type of reaction can be used to deposit a host of materials, as disclosed in U.S. Pat. No. 5,037,775, the disclosure of which is incorporated herein by reference. However, inasmuch as this method depends upon a disproportionation reaction to etch the unwanted nuclei from the masked regions, it is only useful where the deposited material is capable of existing in at least two distinct vapor-phase valence states and one solid-phase valence state.
In U.S. Pat. No. 4,578,142 to Corboy, Jr., et al., a different two-stage process for selective deposition for silicon is disclosed. In the deposition stage, silicon is deposited from a gas mixture which includes hydrogen and a silicon source gas, such as SiH.sub.4, SiH.sub.3 Cl, SiH.sub.2 Cl.sub.2, SiHCl.sub.3 or SiCl.sub.4. In the second stage, the deposited silicon is etched from the masked areas of the substrate in a gas mixture of hydrogen and a silicon etching gas, such as HCl. The two-stage process is cyclically repeated until the desired thickness of silicon is deposited at the unmasked areas of the substrate. The alternating stages of the process are initiated by alternatingly turning on and off the flow of the reducible silicon source gas while simultaneously decreasing and increasing, respectively, the flow of the silicon etching gas.
Like Iyer, et al. and Reisman, Corboy Jr., et al. rely on the preference of the etching reaction for nuclei deposited on the masking layer to achieve the desired result. For this reason, the overall Corboy Jr., et al. process also results in selective deposition. The Corboy, Jr., et al. patent is limited to the deposition of silicon and does not disclose that the disclosed method is useful for depositing any other materials. Again, unlike Reisman who interrupts the flow of the reducing gas while maintaining the flow of the reducible gas, Corboy, Jr. interrupts the flow of the reducible gas and introduces or increases the flow of an etching gas which is a different chemical than either the reducible or reducing gas.